SB-UNIX is an internal implementation package. If you use
functionality provided by it, sooner or later your code will break,
because SBCL changed its internals: It is subject to change without
notice. When we stop using a function that used to live there, that
function gets deleted. Things may also change names and
interfaces.

CL-USER> (documentation (find-package :sb-unix) t)
"private: a wrapper layer for SBCL itself to use when talking
with an underlying Unix-y operating system.
This was a public package in CMU CL, but that was different.
CMU CL's UNIX package tried to provide a comprehensive,
stable Unix interface suitable for the end user.
This package only tries to implement what happens to be
needed by the current implementation of SBCL, and makes
no guarantees of interface stability."

Instead, use either SB-POSIX (which is the supported
external API), or call the foreign functions directly. Alternatively,
if you're using something from SB-UNIX that doesn't have a
counterpart in SB-POSIX or elsewhere, put a feature request / bug
report on Launchpad
explaining what you need. Just saying "wanted: a supported equivalent
of SB-UNIX:UNIX-FOO" is enough.

(The same holds more or less for all internal packages, of course,
but SB-UNIX is the most common offender.)

I realize this is an imperfect world, and sometimes using an
unsupported API is the best thing you can do, but please try to
avoid this especially in libraries used by other people as well.